Method of making a two-phase thermal bag component cooler

ABSTRACT

A two-phase liquid cooling system for an electronic component comprised of flexible sealed bag which is partially filled with a liquid coolant. Sufficient residual non-condensing gas is maintained in the bag so that some of the gas dissolves in the liquid coolant when the device is not operating and at ambient temperature. During warmup, the residual gas comes out of solution and creates nucleation sites that assist in initiating boiling. The bag is air and fluid-impermeable, and has sufficient flexibility such that as coolant vaporizes, the bag expands to maintain the internal bag pressure substantially the same as the ambient environmental pressure. The bag may also be provided with a metal heat spreader plate which passes through a wall of the bag an assists with transferring heat from the component to the coolant. The heat spreader plate may be specially treated to allow the flexible bag material to by directly heat sealed to the plate and to provide nucleation sites to enhance coolant boiling.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of our application Ser. No.08/120,153 filed Sep. 10, 1993 entitled "Two-Phase Component Cooler".

FIELD OF THE INVENTION

This invention relates generally to an apparatus for dissipating theheat generated by electronic components, such as integrated circuits,and, in particular, to a flexible bag-like container which contains acoolant in both a liquid phase and vapor phase to provide enhanced heatdissipation.

BACKGROUND OF THE INVENTION

Traditional methods of cooling electronic systems, such as computers,have most commonly involved air as the heat transfer medium within thesystem. In such systems, heat generated by electronic components istypically transferred to the air surrounding the components and then toan ultimate sink, generally the room in which the electronic system islocated. Heat transfer between the components and the surrounding airmay be enhanced by increasing the surface area of contact between theelectronic components and the air, for example, by using a heat sink,which itself may have fins to further increase the surface areaavailable for heat transfer. Air circulation within the system may occurby natural convection or may be further enhanced by forcing the air tocirculate about the components by means of a fan or blower.

As electronic equipment becomes more sophisticated and yet more compactin size, the density of the heat-dissipating components mounted on aparticular circuit board has necessarily increased. This is also true ofthe integrated circuit packages themselves, with the number and densityof active devices, such as transistors, within a given package steadilyincreasing over time.

Unfortunately, these trends of increasing complexity and decreasing sizehave meant that the amount of heat dissipated within a given volume hasalso increased. With current-day integrated circuit technologies, thetime is fast approaching when conventional air convection coolingmethods, even those using forced air circulation with large heat sinks,will not adequately maintain certain high density integrated circuitpackages within their permissible operating temperature range.

Consider, for example, the Pentium™ series of microprocessor chipsrecently introduced by Intel Corporation of Santa Clara, Calif.Depending on its operating speed, a Pentium™ P-5 chip typicallydissipates on the order of 15 watts in a package which has less than twosquare inches of surface area, and a Pentium™ P-6 chip dissipates from22 to 30 watts. Other microprocessor chips, such as the Alpha™microprocessor recently introduced by Digital Equipment Corporation ofMaynard, Mass. are projected to produce heat dissipations of 25 watts orhigher in the same two square inches in their highest speed versions. Ifconventional air convection cooling techniques are to be used, suchchips would require very high air flow rates over finned heat sinks inorder to maintain the chips in their desired operating temperaturerange.

Although such forced air cooling requirements are technically feasible,practical considerations rule out their use in current day personalcomputer applications. One reason is that the blowers or fans necessaryto generate such a high air flow rate would necessarily create anunacceptable noise level in an operating environment such as an officewhich is expected to remain relatively quiet.

This problem is further exacerbated in applications such as laptop andnotebook computers, where the additional noise and weight of a forcedair cooling system is simply neither practical nor desired.

Consequently, there has been renewed interest in adapting liquid coolingtechniques, which make use of natural convection of a coolant, to theproblem of cooling high-powered integrated circuits. These techniquesgenerally fall into two broad groups, including single-phase andtwo-phase cooling systems. In a single-phase liquid cooling system, thecoolant remains in the liquid phase over the normal and expected systemoperating temperature range. In a two-phase system, the coolant changesfrom the liquid phase to the vapor phase during at least one point inthe normal operating temperature range.

One example of a single-phase liquid cooling system makes use of ahermetic enclosure filled with a high-boiling point liquid coolant whichcompletely encloses the heat generating component. The enclosure mayalso be provided with external fins. Heat is transferred from the heatdissipating component to the liquid coolant by conduction and from thecoolant to the walls of the enclosure by natural convection; theenclosure itself may further be cooled by circulating air around it.Such a cooling method can be effective, but involves other problems suchas chemical incompatibilities between the component and the coolant overthe long-term, and the difficulty of obtaining access to the componentfor maintenance.

Other single-phase systems do not directly immerse the integratedcircuit component in the liquid, but instead confine just the coolant toa container which is then placed in intimate contact with the component.Heat is thus conducted from the component through the container wallinto the liquid, which then dissipates the heat by natural convection.

One embodiment of the latter single-phase system uses a container in theform of a sealed flexible bag which is completely filled with a liquidcoolant. The bag is typically constructed from a flexible plastic filmwhich is relatively impermeable to both the air and the enclosed liquid.Metal inserts or thermal vias, which pass through the wall of the bag,may also be used with this type of system to more efficiently conductthe heat from the component to the coolant. Examples of such coolant bagsystems are shown in U.S. Pat. Nos. 4,997,032 and 5,000,256 assigned toMinnesota Mining and Manufacturing Company of St. Paul, Minn.

While such single-phase bag systems can be useful in certain situations,they have several disadvantages. Because they use a single-phasecoolant, the available heat transfer rate is still relatively low.Consequently, they cannot typically be used with the high heatdissipating electronic components such as microprocessors. In addition,the bags have a relatively large volume which conflicts with the currenttrend of reducing system size as much as possible, and thus single-phasebag systems have not found widespread practical application.

Two-phase liquid cooling systems have increasingly been used to overcomethe limitations associated with single-phase systems. In a two-phasesystem, as the component heats up, a liquid coolant is vaporized. Thevapor then travels to a condenser section of the system, where thecoolant vapor is converted back into a liquid. The liquid is returned bysome means to the heat dissipating component and the boiling andcondensing cycle is continuously repeated.

Such a two-phase device is shown in U.S. Pat. No. 3,741,292 assigned toInternational Business Machines Corporation of Armonk, N.Y. In thatsystem, the heat dissipating component is placed within a hermeticenclosure and directly immersed in a pool of low boiling point,dielectric liquid coolant. The heat dissipated by the component causesthe liquid to boil, and the resulting vapor is collected in an enclosurespace located above the liquid pool. The enclosure space is filled withinwardly extending fins which serve as a condenser for the coolantvapor. As the vapor condenses, it runs back into the liquid pool underthe influence of gravity.

Other two-phase cooling systems, so called heat pipe systems, do notdirectly immerse the component into the coolant. Such systems consist ofan elongated hermetic container made with thermally conductive walls,for example, from copper. One end of the container acts as an evaporatorand the other end acts as a condenser. A wick or other capillary device,such as a fine mesh screen, extends along the interior of the container.During manufacture of the heat pipe, the container is partially filledwith low boiling point liquid coolant, and any residual, non condensinggases, such as air, are purged, and the container is then sealed. Theheat dissipating component is mounted to the evaporator end of the pipe,and heat is transferred by conduction through the container wall. As thecoolant evaporates, or boils, the resulting vapor travels down thecontainer to the other end where it condenses back to a liquid. Theliquid is then returned to the evaporator end by means of the wick.

Although the direct immersion and heat pipe techniques can transfer heataway from the heat dissipating component quite efficiently, they alsohave their limitations. More specifically, both techniques use rigid,hermetically sealed containers. When the ambient temperature changes,the pressure inside the container changes, with a consequent change inthe boiling point of the coolant. Thus, the cooling capacity of thesystem changes when the ambient temperature changes.

In addition, because the container is evacuated, there exists asignificant pressure differential along the walls of the container. Asthe container is exposed to repeated heating and cooling cycles, therepeated change in pressure differential causes the walls of thecontainer to flex. Eventually, the container fatigues, causing smallleaks. When a leak does occur, air is drawn into the container. Lateron, when the component is then reactivated, the presence of airincreases the pressure inside the container and may cause some of theliquid to be driven out of the container, thereby compromising thecooling capability of the device. Consequently, such devices aretypically not considered to be useful in environments where long-termlow-maintenance operation is required.

Furthermore, the change in internal pressure results in a furtherincrease in the coolant boiling point, which may also be altered by thepresence of any residual air introduced into the system by leaks. Suchdevices thus cannot be expected to have a single predictable boilingtemperature, and are therefore difficult to control over a widetemperature range.

Prior art two-phase systems are also prone to a phenomenon calledovershoot. This occurs during device warmup as a result of the fact thatthe coolant does not begin to boil when the device temperature initiallyreaches the nominal boiling point. Instead, the tendency is for thetemperature to continue to increase past the boiling point, and then forboiling to suddenly erupt. Once coolant boiling finally does occur, thedevice temperature returns to its normal operating range. However, inthe meantime, the system has been temporarily subjected to a temperaturewell above the boiling point. Overshoot is a highly undesirablecondition as it stresses the cooling system components and in some casesmay even cause the components to temporarily operate outside theirexpected temperature range.

What is needed is a cooling device which will adequately coolcurrent-day high-powered integrated circuits in a compact, reliablepackage, which avoids the problems associated with prior art systems.

Specifically, the cooling device should have a heat transfer rate higherthan the heat transfer rates available with known single-phase systems,without requiring that the heat-dissipating component be immersed in acoolant liquid. Furthermore, the cooling device should not exhibit theproblems of present day two-phase systems, but rather should be immuneto leaking, have a predictable boiling point, and avoid overshoot.

SUMMARY OF THE INVENTION

Briefly, the inventive liquid cooling device comprises a sealed bag orother closed container formed of a flexible material which isimpermeable to both air and liquid coolant. A portion of the bag isdisposed adjacent to the heat-dissipating component and the interior ofthe bag is partially filled with a liquid coolant having a boiling pointbelow the normal maximum operating temperature of the heat-dissipatingcomponent. Because the bag is only partially filled with coolant, thesystem cycles in two-phase operation, with the coolant alternatelyvaporizing at the bag portion adjacent to the component and condensingat bag portions distant from the component. The portion of the bag whichis not filled with coolant is filled with coolant vapor andnon-condensing gas and the bag is sufficiently flexible such that theinterior bag pressure remains substantially at the ambient environmentalpressure both at ambient temperature and over the entire operatingtemperature range.

In accordance with one preferred arrangement, the flexible bag is madefrom a material which has a sufficient film strength and a sufficientlyhigh melting point to withstand the normal operating pressures andtemperatures produced by the heat-dissipating component. Oneillustrative material suitable for use with the preferred arrangement isa laminate with a central aluminum layer to act as a vapor barrier, onemore outer layers formed of a polyester to provide film strength,structural rigidity, and abrasion resistance, and an inner layer formedof a composite polypropylene for heat sealing. Such laminates, known asMeals Ready to Eat (MRE) materials are well known in the food packagingindustry.

Preferably, the liquid coolant disposed within the bag is athermally-stable, environmentally-safe fluorochemical with a boilingpoint at atmospheric pressure in the vicinity of 60° C. which is wellbelow the maximum operating temperature of 80°-90° C. of mostcommercial-grade electronic components.

In accordance with another embodiment, a heat spreader plate ispreferably disposed between the interior of the bag and theheat-dissipating component, which heat spreader plate acts as a thermalvia between the coolant and the heat-dissipating component. Since it isa primary surface for transfer of heat from the heat-dissipatingcomponent to the coolant, the heat spreader plate provides a stable,predictable surface at which the phase change from liquid to vapor willtake place within the cooling system and, in accordance with anotherembodiment of the invention, the plate is treated to provide nucleationsites to enhance this boiling.

The heat spreader plate is formed of material having high thermalconductivity, such as aluminum, and extends through the wall of the bag.Accordingly, the heat spreader plate has a first portion outside the bagwhich thermally contacts the heat-dissipating component and a secondportion inside the bag which thermally contacts the coolant liquid. Thebag may be sealed around the edges of the heat spreader plate by meansof O-ring or other compression seals, but, in accordance with anotherembodiment, the heat spreader plate is first treated to allow the bagmaterial to be directly bonded to the spreader plate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an isometric view of a liquid cooling apparatus constructedin accordance with the principles of the invention;

FIG. 1B is an isometric view of the inventive cooling apparatus insertedinto an enclosure which holds the apparatus in a vertical orientation;

FIG. 2 is a cross-sectional view of the cooling apparatus shown in FIGS.1A and 1B taken along section line 2--2 shown in FIG. 1A;

FIG. 2A is a cross-sectional view showing in detail a clip for holdingthe cooling apparatus in place against a heat-dissipating component;

FIG. 3 is a cross-sectional view of an alternative embodiment of a heatspreader plate for use with the present invention making use of fins toprovide greater surface area for heat transfer;

FIG. 4 is a top plan view of a heat spreader plate;

FIG. 5 is an alternative arrangement showing the bag disposed in ahorizontal orientation;

FIG. 6 is a cross-sectional view showing integrated circuit chipsdisposed underneath the bag; and

FIGS. 7A, 7B, and 7C are cross-sectional and top views of a pleatedportion of the bag.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1A illustrates a heat transfer apparatus 10 for cooling anelectronic component such as an integrated circuit or chip package 12.The integrated circuit package 12 is mounted on a circuit board 14 ofthe type that is commonly used in computer and other electronic systems.

Integrated circuit package 12 is a relatively high-powered componentwhich may dissipate 10 watts or more and which itself may contain manyhundreds of thousands of active electronic devices. Most commonly, theintegrated circuit package 12 is a microprocessor chip or other suchhighly-sophisticated component, but may also consist of severalintegrated circuits mounted on a heat spreader plate as will hereinafterbe described. In addition, the integrated circuit package may be amulti-chip module having more than one chip within a single ceramic orplastic package.

Components 16 other than the integrated circuit package 12 are alsotypically mounted on the circuit board 14. For clarity, FIG. 1A does notshow that many additional integrated circuit chips are normally includedon the circuit board 14. The circuit board 14 includes connectors 18enabling the integrated circuit package 12 and other components 16 tocommunicate with circuits on other circuit boards.

The circuit board 14 is intended to be mounted in a vertically-orientedposition in a circuit board enclosure 15, as shown generally in FIG. 1B,and the circuit board 14 is typically inserted and extracted from theenclosure 15 using levers 20.

Returning to FIG. 1A, the heat transfer apparatus 10 according to theinvention will be described more particularly. The invention comprises aflexible, sealed bag 22 which is shaped to conform to the availablespace on the circuit board 14 above the integrated circuit package 12,while at the same time having a low enough profile so as not tointerfere with adjacent circuit boards 14 when installed in theenclosure 15 (FIG. 1B). In addition, the bag 22 is shaped so that vaporwhich is generated in the lower evaporator portion 25 of the bag 22condenses in the upper condenser portion 23 and returns to theevaporator portion 25 by gravity.

The bag 22 is typically manufactured separately from the integratedcircuit chip 12, and is formed of a flexible sheet material which isimpermeable to air, other gases, and liquids. The material may be asingle or multi-layer plastic film and is usually a type ofthermoplastic film, because the latter is readily available and is oftenheat-sealable. The preferred material for the bag 22 is a multi-layerfilm formed of a thin aluminum or other metallic layer laminated on bothsides with one or more layers of a modified polyester terphthalate (PET)film and is held together with a laminating adhesive on one side, andpolypropylene film on the other side. One example or such amulti-layered film is sold as Meals Ready to Eat (MRE) type film by theAmerican National Can Company of Mt. Vernon, Ohio. The bag can also bemade of any other similar material which can withstand the normaloperating temperature of the components, typically 70-110 degreescentigrade (°C). The sheet material should be sufficiently flexible thatthe internal pressure in the bag interior remains at, or near, theambient environmental pressure over the entire operating temperaturerange to which the bag is exposed. Since the internal bag pressurechanges little during operation, the coolant boils at, or very near, itsboiling point at ambient pressure. Thus, the device 10 does not have theextended temperature ranges which characterize the prior art devices.

The bag 22 is attached to the circuit board 14 and to the integratedcircuit package 12. To permit attachment of the bag 22 to the circuitboard 14, the periphery 21 of the bag 22 is formed more rigidly than thecenter, and includes holes 24 adapted to receive fasteners such asscrews or the like which hold the periphery 21 of the bag 22 againststandoffs 29. The rigid periphery 21 of the bag 22 may be formed byheat-sealing the two layers of the plastic film together to form a rigidlip or by heat-sealing or attaching the plastic film layers to a thirdmore rigid frame.

The peripheral portions 26 of the bag adjacent the chip 12 are alsosomewhat rigid and adapted and/or shaped to be held in position againstthe integrated circuit package 12 with one or more clips 28. The clipshold the lower side of the bag 22 against the outer surface of thepackage 12 in a manner that will be described in greater detail below.

As shown in the cross-sectional view of FIG. 2, the interior 30 of thebag 22 is partially filled by a quantity of liquid coolant 31. Inparticular, as indicated by level line 21a, the liquid 31 typicallyoccupies from 75 to 90 percent of the bag interior 30 when the bag is instorage, i.e., at room temperature. The bag 22 must only be partiallyfilled with coolant, so that two-phase operation occurs, as describedbelow. However, the bag should be filled with enough coolant so that atleast a portion of the heat spreader 34 is always immersed in liquidduring operation. Thus, even at the maximum operating temperature of thecondenser (e.g., when a maximum amount of coolant is in the gas phase),the liquid level is approximately as shown by level line 21b.

The coolant liquid 31 preferably comprises a liquid (at the ambientenvironmental temperature) which is thermally conductive, chemicallyinert, non-toxic, non-ozone-depleting, and thermally stable, and whichis also preferably electrically non-conductive to prevent shortcircuiting of external electronic components should the bag 22 beruptured for any reason. The thermal stability of the liquid 31 enablesit to maintain its physical and chemical properties throughoutrepetitive thermal cycles of the integrated circuit package 12 duringnormal use.

Preferred liquids include the well-known Fluorinert™ electronic fluidswhich are commercially available from the Minnesota Mining andManufacturing Company of St. Paul, Minn. (Fluorinert is a trademark ofthe Minnesota Mining and Manufacturing Company of St. Paul, Minn.). Acoolant liquid which is particularly suitable for use with the inventionis Fluorinert™ FC-72 liquid, which has a relatively low boiling point ofapproximately 56 degrees Centigrade (°C). The boiling point of FC-72 isthus well below the maximum operating temperature range specified of80°-90° C. for the typical integrated circuit package 12. Other coolants31 which are suitable for use with the invention includes, but is notlimited to methylene chloride, ethyl alcohol, methanol, and otherchemicals, as long as they exhibit a predictable boiling point over arange of operating temperatures.

Contrary to prior art designs, the bag 22 is not purged of residualgases during manufacture. Instead, residual, non-condensing gases, suchas air, are sealed within the system in order to promote boiling, in amanner that is described in detail below.

As temperature inside the bag 22 increases, because the bag 22 is formedof a flexible material, the amount of gas inside the bag is not affectedby the ambient pressure. The bag 22 is thus less prone to the long termleaking problems associated with certain other prior art devices inwhich the bag or other container is evacuated of such gases orcompletely filled with coolant 31. In addition, as explained below, theresidual gases dissolve in the coolant liquid 31 and assist ininitiating boiling.

The process of boiling is of particular concern to the presentinvention, because, unlike prior art arrangements using similar bags, acooling system constructed in accordance with the principles of thepresent invention requires that the liquid 31 enter a vapor state whenthe temperature of the integrated circuit package 12 rises to apredetermined temperature.

More particularly, as heat is dissipated by the integrated circuitpackage 12, the temperature of the coolant 31 approaches the boilingpoint. Once boiling begins, thermal transfer is enhanced (as compared toconvection) since the coolant phase change requires that increasedamounts of heat be transferred to the coolant. The transferred heatenergy is, in turn, transported in the form of coolant vapor into aspace 33 formed above the liquid level 21a, 21b. The vapor in the space33 then eventually condenses along the upper walls of the bag 22 andreturns to a liquid state, where it drops back down the pool of liquid31 by the force of gravity. The coolant thus experiences repeated phasechanges.

Sufficient residual gas is maintained in the system so that some of thegas dissolves in the liquid coolant 31 when the device is not operatingand is at ambient temperature. During warmup, the residual gas comes outof solution and creates nucleation sites that assist in initiatingboiling and thus minimize the aforementioned "overshoot" phenomenon.

As shown in FIG. 2, bag 22 may not be directly attached to a plasticintegrated circuit 12. Rather, a heat spreader plate 34 can bepositioned to contact the integrated circuit package 12 substantiallyalong one of the major outer surfaces thereof. Spreader plate 34 can bemanufactured as an integral part of the integrated circuit 12 package ormanufactured separately. Alternatively, one or more individualintegrated circuit devices can be mounted directly on the heat spreaderplate 34 opposite the bag 22.

The heat spreader plate 34 extends through a hole in the bag 22, andprovides a high thermal conductivity heat transfer path or via directlybetween the integrated circuit package 12 and the coolant 31. As such,the transfer of heat into the coolant 31 is not hindered by any thermalresistance of the bag 22 in areas adjacent the integrated circuitpackage 12. Moreover, since only the heat spreader plate 34, and not theouter surface of the integrated circuit package 12 contacts the coolant31, the integrated circuit package 12 need not be compatible with longterm exposure to the coolant fluids used within the bag 22.

The heat spreader plate 34 is typically made of a material having highheat conductivity such as copper, diamond-copper composite, aluminum,or, in some cases, high-conductivity plastic.

Additional nucleation sites to enhance coolant boiling may be providedon the surface of the heat spreader plate 34 by specially treating theinner surface of the heat spreader plate 34 (i.e., the surface exposedto the liquid) to provide a pitted surface. If the pits are of suitablesize and shape, they will provide nucleation sites which ensure that thecoolant 31 boils substantially at its boiling point.

The heat spreader plate 34 may be mounted to the integrated circuitpackage 12 using epoxy, solder, cement, compression seals, thermalgrease or other arrangements, and may be held in place against the chip12 using contact springs or clips 28, as previously described. FIG. 2Ais a more detailed view of a preferred C-shaped clip 28, as arranged tohold the bag 22, heat spreader 34 and package 12 together. A T-shapedchannel 38 is formed in the outer periphery of the lower portion of thespreader plate 34, and is adapted to receive the clip 28, which in turnhas a T-shaped end 40 adapted to fit into the channel 38. The clip 28 isthereby fastened to the plate 34 and fits over the lower edge of thechip package 12, holding the assembly against the surface of chippackage 12.

The heat spreader plate 34 may be smooth or flat for cooling relativelylower power components. However pin fins or tab (continuous) fins may beformed in the heat spreader plate 34 to help draw the coolant into thehottest areas of the spreader plate 34. For example, in the design shownin FIG. 3, heat spreader plate 34 has pin fins 37 formed thereon on aninner surface 50 which faces into the bag interior. In such anapplication, of course, the bag 22 might have a larger cross dimensionin order to accommodate the pins 37.

Another possibility is to provide a spreader plate as shown in FIG. 4.In that arrangement, a raised portion 44 of the plate 34 extends intothe fluid 31, and may have slots 42 cut into it. This further increasesthe surface area and makes additional sites available for boiling. Thesizes of the slots 42 are chosen to optimize bubble formation andcoolant return.

Returning attention briefly to FIG. 2, the bag 22 is attached in asuitable manner, such as, preferably by heat-sealed directly to the heatspreader plate 34. To insure a reliable heat seal, the heat spreaderplate 34 may be specially treated before heat-sealing is performed. Inaccordance with another embodiment of the invention, the heat spreaderplate 34 is formed of aluminum. The aluminum is treated by aconventional anodizing process. However, after the spreader plate 34 hasbeen anodized, the nickel acetate or other sealing solution that istypically applied to seal the anodized surface is not applied. Theunsealed, anodized aluminum plate 34 thus provides a porous surface forbetter adherence of the bag 22 with small cracks and otherirregularities in the surface providing sites for the meltedthermoplastic to adhere. The porous surface presented by the anodizedaluminum also provides additional nucleation sites to further assist inencouraging boiling at an early stage.

Instead of heat-sealing the bag 22 to the plate 34, conventional O-ringarrangements may also be used to hold the bag 22 in compression sealagainst the surface of the heat spreader plate 34.

In another embodiment, the aluminum surface may be treated with anorganic solvent primer such as that sold under the trademark Morprime10-B, by Morton International, Inc. of Chicago, Ill.. Such primersprovide good adhesion for the polypropylene bag material.

The heat spreader plate 34 may also be formed of sintered metal. It hasbeen found that if the sintered metal parts are compacted toapproximately 60%-70% density to produce an average pore size on theorder of 100 microns, the pores will produce additional nucleation sitesto encourage early boiling and discourage overshoot.

Other orientations of the bag 21 are also possible. For example, insteadof being clipped to package 12, the bag 22 may be mounted over package12 by placing standoffs 29 on both sides of the package 12. Thisembodiment, as shown cross section in FIG. 5, is of particular use inapplications where the printed circuit board 14 cannot be guaranteed tobe placed in a vertical orientation, such as in laptop or desktopcomputers. In this embodiment, the bag 22 is simply pressed in positionover the integrated circuit package 12.

In general, wicking is typically not required to reliably return thecondensed coolant to the liquid coolant pool, and that gravity canusually be relied upon for this purpose. However, there can be certaininstances, as shown in FIG. 6, in which wicking material 60 may beprovided inside the bag 22 along the interior bag surfaces, with certainadvantages. Such an arrangement is particularly convenient inarrangements where the outer surface 52 of the bag 22 contactsadditional heat-dissipating components 55 which may be mounted on thecircuit board 14 beneath the bag 22. The wick 60 can typically beprovided as a polyester or polypropylene woven fabric, or as a metalscreen.

When present, the wick 60 pulls liquid coolant by capillary action upalong the upper surfaces of the bag 22 adjacent the additional chips 55which may need some cooling but not quite as much cooling as the primarychip package 12.

However, when a wick is used the coolant preferably has a high surfacetension. The aforementioned preferred Fluorinert™ material has a verylow surface tension, and does not wick particularly well, so that othercooling fluids such as water, methanol, or acetone are preferred in thisapplication.

It has been found that over time, depending upon the material chosen forthe bag 22, that the bag 22 may tend to sag under the force of gravity,so that the interior area of the bag 22 adjacent the spreader 34 mayincrease, causing the minimum coolant level 21b to decrease (FIG. 2).Thus, it is desirable in some instances to pre-form pleats in the bag22, as shown in the cross sectional view of FIG. 7A, to minimize theeffects of such sagging. The pleats, which act as structural stiffeners,may be circular or square in shape, as shown in FIG. 7B and 7Crespectively, or may be of other shapes.

Heat sealed dimples may also be used to physically attach the film tothe heat spreader 34 to prevent sagging.

There are several advantages of this invention which will now beunderstood by those of skill in the art. For example, the single bag 22provides two-phase cooling, without the need for a separate condenserand evaporator unit, tubes or other interconnecting apparatus. Inaddition, the cooling system 10 does not require fans, blowers or othernoise generating components and thus is ideal for use in applicationswhere noise is of prime consideration such as offices and the like.Further, the bag 22 has a low profile, and is lighter than the usualmetal heat sinks associated with large integrated circuit chips 12.Finally, the use of the polypropylene bag 22 in connection with ananodized aluminum heat transfer plate 34 provides good and reliable heatseal connection, thereby providing an enclosure space 30 which ispredictably free from leaks and other failures, and thus requires nomaintenance over the long term.

The advantages of two-phase cooling are provided in an arrangement inwhich single-phase cooling has heretofore only thought been possible.The invention is adaptable to a wide variety of devices including laptopcomputers, server computers and other applications in which quiet,inexpensive and reliable cooling of integrated circuit chips isimportant. For example, a cooling system built in accordance with theprinciples of the invention in the manner shown in FIGS. 1A and 1B hasbeen found sufficient to cool a 42 watt chip package 12 to a temperatureof 57 degrees centigrade which is far below the 70 degree mean operatingtemperature specified for the Pentium™ chip by Intel Corporation.

The terms and expressions which have been employed above are used asterms of description and not of limitation, and there is no intention,in the use of such terms and expressions, of excluding any equivalentsof the features shown and described or portions thereof, but it isrecognized that various modifications are possible within the scope ofthe invention claimed.

What is claimed is:
 1. A method for fabricating a cooling apparatuscomprising a closed bag having at least one wall of flexible,thermoplastic sheet material with a hole therein, the hole having aperiphery, and an aluminum plate having a surface, the method comprisingthe steps of:A. anodizing without sealing the aluminum plate surface toproduce a porous anodized plate surface; and B. adhering the sheetmaterial at the hole periphery to the porous anodized plate surface toproduce a sealed bag.
 2. A method as in claim 1 wherein step B comprisesthe step of:B1. heat sealing the hole periphery to the porous anodizedplate surface.
 3. A method as in claim 1 wherein step B comprises thestep of:B2. adhering the hole periphery to the porous anodized platesurface with an adhesive adapted for adhering thermoplastic material toaluminum.
 4. A method as in claim 1 wherein step B comprises the stepsof;B3. priming the aluminum plate with Morprime 10-B, and B4. anodizingthe primed plate.
 5. A method for fabricating a cooling apparatusaccording to claim 1 wherein step A comprises the steps of:A1. selectinga first sheet and a second sheet of a multi-layer laminated material,each of the first and second sheets of laminated material having aperiphery; and A2. heat-sealing the peripheries of the first and secondsheets of laminated material together to form the bag.
 6. A method forfabricating a cooling apparatus comprising the steps of:A. fabricating aclosed bag having at least one wall of flexible, thermoplastic sheetmaterial with a hole therein, the hole having a periphery; B.fabricating a bare aluminum plate having a sealing surface; C. anodizingthe aluminum plate sealing surface to produce a porous anodized platesealing surface; D. positioning the porous anodized plate sealingsurface in contact with thermoplastic sheet material at the holeperiphery; and E. adhering the thermoplastic sheet material at the holeperiphery to the porous anodized plate sealing surface.
 7. A method forfabricating a cooling apparatus according to claim 6 wherein step Bcomprises the step of:B1. fabricating the aluminum plate with tinnedsurfaces located at areas apart from the sealing surface.